1. Field of the Invention
The present invention relates to a process for the precipitation of the amino acid cystine from a solution in sulfuric acid.
2. The Prior Art
The amino acid cystine is usually obtained by acid hydrolysis of keratinous waste products such as, for example, human hair, and is purified by reprecipitation. Reprecipitatibn from solution in hydrochloric acid is well known in this field of technology. Multiple application of reprecipitation allows a crystalline product to be isolated which meets the purity requirements for the target markets (foodstuff specifications such as, for example, FCC IV for USA and drug specifications such as, for example, USP 23 for USA, Ph.Eur.2000 for Europe). This usually entails initially the cystine solution in (hydrochloric) acid being brought to the precipitation temperature, and then the base, usually aqueous ammonia or aqueous sodium hydroxide solution, being metered in until the pH is at a defined value (mostly between 0.6 and 3.5).
Although application of this process to a solution of cystine in sulfuric acid likewise results in very pure cystine, it always comprises a certain residual sulfate content. This is, depending on the experimental conditions, between 400 and 1500 ppm. The maximum residual sulfate permitted required by Ph. Eur. must, however, be only 300 ppm.
It is an object of the present invention to provide a process which makes it possible to precipitate solid crystals of cystine which has a residual sulfate content of less than 300 ppm, which additionally meets all the other purity criteria of the foodstuff and drug specifications applicable to a solution in sulfuric acid.
The above object is achieved by the present invention which relates to a process for precipitating solid crystals of cystine which comprises forming a mixture of a solution of cystine in aqueous sulfuric acid and of an aqueous solution of a base by simultaneous metering into a mixing container the solution of cystine and the solution of the base; the metering taking place in such a way that the mixture in the container has a pH between 1.0 and 7.0 and a temperature between 30xc2x0 C. and the boiling point of the mixture; and precipitating solid crystals of cystine.
The concentration of cystine in the added solution is between 20 g/l and 200 g/l, preferably 40 g/l to 180 g/l, particularly preferably 50 to 150 g/l.
The molar excess of sulfuric acid in the added solution based on cystine is chosen to be as small as possible so that the H2SO4:cystine molar ratio is preferably from 1.5 to 3.0, particularly preferably 2.0 to 2.5.
The aqueous solution of a base is preferably selected from the group consisting of an aqueous ammonia solution, an aqueous solution of an alkali metal hydroxide, an aqueous solution of an alkali metal bicarbonate, an aqueous solution of an alkali metal carbonate or a mixture of these solutions.
An aqueous solution of ammonia, an aqueous solution of sodium hydroxide or an aqueous solution of potassium hydroxide is preferably used.
The concentration of the base can be varied within a wide range and is preferably between 10% and 50% by weight based upon the total weight of the aqueous solution of the base.
The pH is kept constant within a range from 1.0 to 7.0, preferably 1.5 to 5.0, particularly preferably 1.8to 3.5, during the precipitation by adjusting the metering rates.
The variation between the lowest and highest pH in the mixture during a single precipitation is preferably less than 3 pH units.
The temperature during the precipitation is kept at between 30xc2x0 C. and the boiling point of the mixture, preferably between 40xc2x0 C. and the boiling point of the mixture, particularly preferably between 50xc2x0 C. and the boiling point of the mixture.
Precipitation of the solid crystals of cystine can be carried out both batchwise and continuously.
In a batchwise management of the process, firstly a small amount of water or mother liquor from a preceding precipitation is introduced into the mixing container. After the contents of the mixing container have been brought to the precipitation temperature, the components are then metered in as described. A particularly slow metering generally has beneficial effects on the purity of the product. However for economic reasons, the total duration of the metering is to be chosen preferably between 0.5 and 6 h, particularly preferably between 1 and 3 h.
The components may however be heated individually to the precipitation temperature before being introduced into a mixing container, or be brought to the desired precipitation temperature only when in the mixing container.
In a continuous procedure, the components are individually heated to the precipitation temperature before being introduced into the mixing container, and are then metered into the mixture.
After the precipitation of the solid crystals of cystine has been carried out, the crystals are removed from the mother liquor by a solid/liquid separation such as, for example, centrifugation, filtration or sedimentation of the mixture.
Preferred solid/liquid separations are centrifugation and filtration.
The separation of the solid crystals of cystine from the mother liquor can be carried but as is known for the separation after precipitation from solution in hydrochloric acid. It is an advantage of the process of the invention that no corrosion problems occur as do result with the chloride-containing, acid solutions from the prior art.
The solid crystals of cystine are then washed with water, preferably with deionized water. The amount of water is such that sulfate is no longer detectable in the filtrate which drains off using a 10% strength barium chloride aqueous solution. From 5 to 20 volumes of water based on the dry weight of the crystals are preferably employed. The amount of water is moreover preferably divided into from 3 to 20 portions.
Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying Examples which disclose several embodiments of the present invention. It should be understood, however, that the Examples are designed for the purpose of illustration only and not as a definition of the limits of the invention.